Multiple adaptive loop filter sets for video coding

ABSTRACT

Techniques for signaling and decoding adaptive loop filter information is described. A video decoder may be configured to decode a block of the video data, and receive an adaptation parameter sets (APS) in an encoded video bitstream for the block of the video data, wherein the APS includes a plurality of adaptive loop filter sets for luma components of the block of the video data. The video coder may determine an adaptive loop filter from the plurality of adaptive loop filter sets in the APS to apply to the decoded block of the video data, and apply the determined adaptive loop filter to the decoded block of the video data to create a filtered block of the video data.

This application claims the benefit of U.S. Provisional Application No.62/849,606, filed May 17, 2019, the entire content of which isincorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to video encoding and video decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, and the like. Digital video devices implement videocoding techniques, such as those described in the standards defined byMPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), andextensions of such standards. The video devices may transmit, receive,encode, decode, and/or store digital video information more efficientlyby implementing such video coding techniques.

Video coding techniques include spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (e.g., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to ascoding tree units (CTUs), coding units (CUs) and/or coding nodes. Videoblocks in an intra-coded (I) slice of a picture are encoded usingspatial prediction with respect to reference samples in neighboringblocks in the same picture. Video blocks in an inter-coded (P or B)slice of a picture may use spatial prediction with respect to referencesamples in neighboring blocks in the same picture or temporal predictionwith respect to reference samples in other reference pictures. Picturesmay be referred to as frames, and reference pictures may be referred toas reference frames.

SUMMARY

In general, this disclosure describes techniques related to adaptiveloop filters (ALFs). In particular, this disclosure describes varioustechniques for signaling and decoding luma and/or chroma adaptive loopfilters for blocks of video data (e.g., coding tree blocks). In someexamples, this disclosure describes techniques for signaling anddecoding adaptive loop filter information, including adaptive loopfilter information for multiple adaptive loop filter sets, in one ormore adaptation parameter sets (APSs). The adaptive loop filterinformation may include one or more of adaptive loop filter sets,adaptive loop filter coefficients, merging tables, and/or clippingvalues. The techniques of the disclosure may improve coding efficiencyand/or reduce distortion in decoded/reconstructed video data byproviding additional flexibility in selecting possible adaptive loopfilters to apply for both luma and chroma components of video data.

The techniques of this disclosure may be applied to extensions to any ofthe existing video codecs, such as extensions to High Efficiency VideoCoding (HEVC), or part of standards currently being developed, such asVersatile Video Coding (VVC), and to other future video codingstandards.

In one example, this disclosure describes an apparatus configured todecode video data, the apparatus comprising a memory configured to storethe video data, and one or more processors in communication with thememory, the one or more processors configured to decode a block of thevideo data, receive an adaptation parameter sets (APS) in an encodedvideo bitstream for the block of the video data, wherein the APSincludes a plurality of adaptive loop filter sets for luma components ofthe block of the video data, determine an adaptive loop filter from theplurality of adaptive loop filter sets in the APS to apply to thedecoded block of the video data, and apply the determined adaptive loopfilter to the decoded block of the video data to create a filtered blockof the video data.

In another example, this disclosure describes a method for decodingvideo data, the method comprising decoding a block of the video data,receiving an APS in an encoded video bitstream for the block of thevideo data, wherein the APS includes a plurality of adaptive loop filtersets for luma components of the block of the video data, determining anadaptive loop filter from the plurality of adaptive loop filter sets inthe APS to apply to the decoded block of the video data, and applyingthe determined adaptive loop filter to the decoded block of the videodata to create a filtered block of the video data.

In another example, this disclosure describes an apparatus configured toencode video data, the apparatus comprising a memory configured to storethe video data, and one or more processors in communication with thememory, the one or more processors configured to encode a block of thevideo data, reconstruct the block of the video data, apply an adaptiveloop filter to the reconstructed block of the video data to create afiltered block of the video data, and signal an APS in an encoded videobitstream for the block of the video data, wherein the APS includes aplurality of adaptive loop filter sets, including the applied adaptiveloop filter, for luma components of the block of the video data.

In another example, this disclosure describes a method of encoding videodata, the method comprising encoding a block of the video data,reconstructing the block of the video data, applying an adaptive loopfilter to the reconstructed block of the video data to create a filteredblock of the video data, and signaling an APS in an encoded videobitstream for the block of the video data, wherein the APS includes aplurality of adaptive loop filter sets, including the applied adaptiveloop filter, for luma components of the block of the video data.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may perform the techniques of this disclosure.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure, and a corresponding coding tree unit(CTU).

FIG. 3 is a block diagram illustrating an example video encoder that mayperform the techniques of this disclosure.

FIG. 4 is a block diagram illustrating an example video decoder that mayperform the techniques of this disclosure.

FIG. 5 is a conceptual diagram illustrating example adaptive loop filtersupports.

FIG. 6 is a conceptual diagram illustrating an example 5×5diamond-shaped filter support.

FIG. 7 is a conceptual diagram illustrating example geometrytransformations.

FIG. 8 is a flowchart illustrating an example encoding method of thedisclosure.

FIG. 9 is a flowchart illustrating an example encoding method of thedisclosure in more detail.

FIG. 10 is a flowchart illustrating an example decoding method of thedisclosure.

FIG. 11 is a flowchart illustrating an example decoding method of thedisclosure in more detail.

DETAILED DESCRIPTION

In general, this disclosure describes techniques related to adaptiveloop filters (ALFs). In particular, this disclosure describes varioustechniques for signaling and decoding luma and/or chroma adaptive loopfilters for blocks of video data (e.g., coding tree blocks). In someexamples, this disclosure describes techniques for signaling anddecoding adaptive loop filter information, including adaptive loopfilter information for multiple adaptive loop filter sets, in one ormore adaptation parameter sets (APSs). The adaptive loop filterinformation may include one or more of adaptive loop filter sets,adaptive loop filter coefficients, merging tables, and/or clippingvalues. The techniques of the disclosure may improve coding efficiencyand/or reduce distortion in decoded/reconstructed video data byproviding additional flexibility in selecting possible adaptive loopfilters to apply for both luma and chroma components of video data.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 100 that may perform the techniques of this disclosure.The techniques of this disclosure are generally directed to coding(encoding and/or decoding) video data. In general, video data includesany data for processing a video. Thus, video data may include raw,unencoded video, encoded video, decoded (e.g., reconstructed) video, andvideo metadata, such as signaling data.

As shown in FIG. 1, system 100 includes a source device 102 thatprovides encoded video data to be decoded and displayed by a destinationdevice 116, in this example. In particular, source device 102 providesthe video data to destination device 116 via a computer-readable medium110. Source device 102 and destination device 116 may comprise any of awide range of devices, including desktop computers, notebook (i.e.,laptop) computers, tablet computers, set-top boxes, telephone handsetssuch smartphones, televisions, cameras, display devices, digital mediaplayers, video gaming consoles, video streaming device, or the like. Insome cases, source device 102 and destination device 116 may be equippedfor wireless communication, and thus may be referred to as wirelesscommunication devices.

In the example of FIG. 1, source device 102 includes video source 104,memory 106, video encoder 200, and output interface 108. Destinationdevice 116 includes input interface 122, video decoder 300, memory 120,and display device 118. In accordance with this disclosure, videoencoder 200 of source device 102 and video decoder 300 of destinationdevice 116 may be configured to apply the techniques for adaptive loopfiltering. Thus, source device 102 represents an example of a videoencoding device, while destination device 116 represents an example of avideo decoding device. In other examples, a source device and adestination device may include other components or arrangements. Forexample, source device 102 may receive video data from an external videosource, such as an external camera. Likewise, destination device 116 mayinterface with an external display device, rather than including anintegrated display device.

System 100 as shown in FIG. 1 is merely one example. In general, anydigital video encoding and/or decoding device may perform techniques foradaptive loop filtering. Source device 102 and destination device 116are merely examples of such coding devices in which source device 102generates coded video data for transmission to destination device 116.This disclosure refers to a “coding” device as a device that performscoding (encoding and/or decoding) of data. Thus, video encoder 200 andvideo decoder 300 represent examples of coding devices, in particular, avideo encoder and a video decoder, respectively. In some examples,devices 102, 116 may operate in a substantially symmetrical manner suchthat each of devices 102, 116 include video encoding and decodingcomponents. Hence, system 100 may support one-way or two-way videotransmission between video devices 102, 116, e.g., for video streaming,video playback, video broadcasting, or video telephony.

In general, video source 104 represents a source of video data (i.e.,raw, unencoded video data) and provides a sequential series of pictures(also referred to as “frames”) of the video data to video encoder 200,which encodes data for the pictures. Video source 104 of source device102 may include a video capture device, such as a video camera, a videoarchive containing previously captured raw video, and/or a video feedinterface to receive video from a video content provider. As a furtheralternative, video source 104 may generate computer graphics-based dataas the source video, or a combination of live video, archived video, andcomputer-generated video. In each case, video encoder 200 encodes thecaptured, pre-captured, or computer-generated video data. Video encoder200 may rearrange the pictures from the received order (sometimesreferred to as “display order”) into a coding order for coding. Videoencoder 200 may generate a bitstream including encoded video data.Source device 102 may then output the encoded video data via outputinterface 108 onto computer-readable medium 110 for reception and/orretrieval by, e.g., input interface 122 of destination device 116.

Memory 106 of source device 102 and memory 120 of destination device 116represent general purpose memories. In some example, memories 106, 120may store raw video data, e.g., raw video from video source 104 and raw,decoded video data from video decoder 300. Additionally oralternatively, memories 106, 120 may store software instructionsexecutable by, e.g., video encoder 200 and video decoder 300,respectively. Although shown separately from video encoder 200 and videodecoder 300 in this example, it should be understood that video encoder200 and video decoder 300 may also include internal memories forfunctionally similar or equivalent purposes. Furthermore, memories 106,120 may store encoded video data, e.g., output from video encoder 200and input to video decoder 300. In some examples, portions of memories106, 120 may be allocated as one or more video buffers, e.g., to storeraw, decoded, and/or encoded video data.

Computer-readable medium 110 may represent any type of medium or devicecapable of transporting the encoded video data from source device 102 todestination device 116. In one example, computer-readable medium 110represents a communication medium to enable source device 102 totransmit encoded video data directly to destination device 116 inreal-time, e.g., via a radio frequency network or computer-basednetwork. Output interface 108 may modulate a transmission signalincluding the encoded video data, and input interface 122 may modulatethe received transmission signal, according to a communication standard,such as a wireless communication protocol. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 102 to destination device 116.

In some examples, source device 102 may output encoded data from outputinterface 108 to storage device 112. Similarly, destination device 116may access encoded data from storage device 112 via input interface 122.Storage device 112 may include any of a variety of distributed orlocally accessed data storage media such as a hard drive, Blu-ray discs,DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or anyother suitable digital storage media for storing encoded video data.

In some examples, source device 102 may output encoded video data tofile server 114 or another intermediate storage device that may storethe encoded video generated by source device 102. Destination device 116may access stored video data from file server 114 via streaming ordownload. File server 114 may be any type of server device capable ofstoring encoded video data and transmitting that encoded video data tothe destination device 116. File server 114 may represent a web server(e.g., for a website), a File Transfer Protocol (FTP) server, a contentdelivery network device, or a network attached storage (NAS) device.Destination device 116 may access encoded video data from file server114 through any standard data connection, including an Internetconnection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., DSL, cable modem, etc.), or acombination of both that is suitable for accessing encoded video datastored on file server 114. File server 114 and input interface 122 maybe configured to operate according to a streaming transmission protocol,a download transmission protocol, or a combination thereof.

Output interface 108 and input interface 122 may represent wirelesstransmitters/receiver, modems, wired networking components (e.g.,Ethernet cards), wireless communication components that operateaccording to any of a variety of IEEE 802.11 standards, or otherphysical components. In examples where output interface 108 and inputinterface 122 comprise wireless components, output interface 108 andinput interface 122 may be configured to transfer data, such as encodedvideo data, according to a cellular communication standard, such as 4G,4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In someexamples where output interface 108 comprises a wireless transmitter,output interface 108 and input interface 122 may be configured totransfer data, such as encoded video data, according to other wirelessstandards, such as an IEEE 802.11 specification, an IEEE 802.15specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. Insome examples, source device 102 and/or destination device 116 mayinclude respective system-on-a-chip (SoC) devices. For example, sourcedevice 102 may include an SoC device to perform the functionalityattributed to video encoder 200 and/or output interface 108, anddestination device 116 may include an SoC device to perform thefunctionality attributed to video decoder 300 and/or input interface122.

The techniques of this disclosure may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, Internet streaming videotransmissions, such as dynamic adaptive streaming over HTTP (DASH),digital video that is encoded onto a data storage medium, decoding ofdigital video stored on a data storage medium, or other applications.

Input interface 122 of destination device 116 receives an encoded videobitstream from computer-readable medium 110 (e.g., storage device 112,file server 114, or the like). The encoded video bitstreamcomputer-readable medium 110 may include signaling information definedby video encoder 200, which is also used by video decoder 300, such assyntax elements having values that describe characteristics and/orprocessing of video blocks or other coded units (e.g., slices, pictures,groups of pictures, sequences, or the like). Display device 118 displaysdecoded pictures of the decoded video data to a user. Display device 118may represent any of a variety of display devices such as a liquidcrystal display (LCD), a plasma display, an organic light emitting diode(OLED) display, or another type of display device.

Although not shown in FIG. 1, in some examples, video encoder 200 andvideo decoder 300 may each be integrated with an audio encoder and/oraudio decoder, and may include appropriate MUX-DEMUX units, or otherhardware and/or software, to handle multiplexed streams including bothaudio and video in a common data stream. If applicable, MUX-DEMUX unitsmay conform to the ITU H.223 multiplexer protocol, or other protocolssuch as the user datagram protocol (UDP).

Video encoder 200 and video decoder 300 each may be implemented as anyof a variety of suitable encoder and/or decoder circuitry, such as oneor more microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), discrete logic, software, hardware, firmware or anycombinations thereof. When the techniques are implemented partially insoftware, a device may store instructions for the software in asuitable, non-transitory computer-readable medium and execute theinstructions in hardware using one or more processors to perform thetechniques of this disclosure. Each of video encoder 200 and videodecoder 300 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device. A device including video encoder 200 and/orvideo decoder 300 may comprise an integrated circuit, a microprocessor,and/or a wireless communication device, such as a cellular telephone.

Video encoder 200 and video decoder 300 may operate according to a videocoding standard, such as ITU-T H.265, also referred to as HighEfficiency Video Coding (HEVC) or extensions thereto, such as themulti-view and/or scalable video coding extensions. Alternatively, videoencoder 200 and video decoder 300 may operate according to otherproprietary or industry standards, such as the Joint Exploration TestModel (JEM) or ITU-T H.266, also referred to as Versatile Video Coding(VVC). A recent draft of the VVC standard is described in Bross, et al.“Versatile Video Coding (Draft 5),” Joint Video Experts Team (WET) ofITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 14^(th) Meeting: Geneva,CH, 19-27 Mar. 2019, JVET-N1001-v5 (hereinafter “VVC Draft 5”). Thetechniques of this disclosure, however, are not limited to anyparticular coding standard.

In general, video encoder 200 and video decoder 300 may performblock-based coding of pictures. The term “block” generally refers to astructure including data to be processed (e.g., encoded, decoded, orotherwise used in the encoding and/or decoding process). For example, ablock may include a two-dimensional matrix of samples of luminanceand/or chrominance data. In general, video encoder 200 and video decoder300 may code video data represented in a YUV (e.g., Y, Cb, Cr) format.That is, rather than coding red, green, and blue (RGB) data for samplesof a picture, video encoder 200 and video decoder 300 may code luminanceand chrominance components, where the chrominance components may includeboth red hue and blue hue chrominance components. In some examples,video encoder 200 converts received RGB formatted data to a YUVrepresentation prior to encoding, and video decoder 300 converts the YUVrepresentation to the RGB format. Alternatively, pre- andpost-processing units (not shown) may perform these conversions.

This disclosure may generally refer to coding (e.g., encoding anddecoding) of pictures to include the process of encoding or decodingdata of the picture. Similarly, this disclosure may refer to coding ofblocks of a picture to include the process of encoding or decoding datafor the blocks, e.g., prediction and/or residual coding. An encodedvideo bitstream generally includes a series of values for syntaxelements representative of coding decisions (e.g., coding modes) andpartitioning of pictures into blocks. Thus, references to coding apicture or a block should generally be understood as coding values forsyntax elements forming the picture or block.

HEVC defines various blocks, including coding units (CUs), predictionunits (PUs), and transform units (TUs). According to HEVC, a video coder(such as video encoder 200) partitions a coding tree unit (CTU) into CUsaccording to a quadtree structure. That is, the video coder partitionsCTUs and CUs into four equal, non-overlapping squares, and each node ofthe quadtree has either zero or four child nodes. Nodes without childnodes may be referred to as “leaf nodes,” and CUs of such leaf nodes mayinclude one or more PUs and/or one or more TUs. The video coder mayfurther partition PUs and TUs. For example, in HEVC, a residual quadtree(RQT) represents partitioning of TUs. In HEVC, PUs representinter-prediction data, while TUs represent residual data. CUs that areintra-predicted include intra-prediction information, such as anintra-mode indication.

As another example, video encoder 200 and video decoder 300 may beconfigured to operate according to VVC. According to VVC, a video coder(such as video encoder 200) partitions a picture into a plurality ofcoding tree units (CTUs). Video encoder 200 may partition a CTUaccording to a tree structure, such as a quadtree-binary tree (QTBT)structure or Multi-Type Tree (MTT) structure. The QTBT structure removesthe concepts of multiple partition types, such as the separation betweenCUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a firstlevel partitioned according to quadtree partitioning, and a second levelpartitioned according to binary tree partitioning. A root node of theQTBT structure corresponds to a CTU. Leaf nodes of the binary treescorrespond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using aquadtree (QT) partition, a binary tree (BT) partition, and one or moretypes of triple tree (TT) (also called ternary tree (TT)) partitions. Atriple or ternary tree partition is a partition where a block is splitinto three sub-blocks. In some examples, a triple or ternary treepartition divides a block into three sub-blocks without dividing theoriginal block through the center. The partitioning types in MTT (e.g.,QT, BT, and TT), may be symmetrical or asymmetrical.

In some examples, video encoder 200 and video decoder 300 may use asingle QTBT or MTT structure to represent each of the luminance andchrominance components, while in other examples, video encoder 200 andvideo decoder 300 may use two or more QTBT or MTT structures, such asone QTBT/MTT structure for the luminance component and another QTBT/MTTstructure for both chrominance components (or two QTBT/MTT structuresfor respective chrominance components).

Video encoder 200 and video decoder 300 may be configured to usequadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, orother partitioning structures. For purposes of explanation, thedescription of the techniques of this disclosure is presented withrespect to QTBT partitioning. However, it should be understood that thetechniques of this disclosure may also be applied to video codersconfigured to use quadtree partitioning, or other types of partitioningas well.

In some examples, a CTU includes a coding tree block (CTB) of lumasamples, two corresponding CTBs of chroma samples of a picture that hasthree sample arrays, or a CTB of samples of a monochrome picture or apicture that is coded using three separate color planes and syntaxstructures used to code the samples. A CTB may be an N×N block ofsamples for some value of N such that the division of a component intoCTBs is a partitioning. A component is an array or single sample fromone of the three arrays (luma and two chroma) that compose a picture in4:2:0, 4:2:2, or 4:4:4 color format or the array or a single sample ofthe array that compose a picture in monochrome format. In some examples,a coding block is an M×N block of samples for some values of M and Nsuch that a division of a CTB into coding blocks is a partitioning.

The blocks (e.g., CTUs or CUs) may be grouped in various ways in apicture. As one example, a brick may refer to a rectangular region ofCTU rows within a particular tile in a picture. A tile may be arectangular region of CTUs within a particular tile column and aparticular tile row in a picture. A tile column refers to a rectangularregion of CTUs having a height equal to the height of the picture and awidth specified by syntax elements (e.g., such as in a picture parameterset). A tile row refers to a rectangular region of CTUs having a heightspecified by syntax elements (e.g., such as in a picture parameter set)and a width equal to the width of the picture.

In some examples, a tile may be partitioned into multiple bricks, eachof which may include one or more CTU rows within the tile. A tile thatis not partitioned into multiple bricks may also be referred to as abrick. However, a brick that is a true subset of a tile may not bereferred to as a tile.

The bricks in a picture may also be arranged in a slice. A slice may bean integer number of bricks of a picture that may be exclusivelycontained in a single network abstraction layer (NAL) unit. In someexamples, a slice includes either a number of complete tiles or only aconsecutive sequence of complete bricks of one tile.

This disclosure may use “N×N” and “N by N” interchangeably to refer tothe sample dimensions of a block (such as a CU or other video block) interms of vertical and horizontal dimensions, e.g., 16×16 samples or 16by 16 samples. In general, a 16×16 CU will have 16 samples in a verticaldirection (y=16) and 16 samples in a horizontal direction (x=16).Likewise, an N×N CU generally has N samples in a vertical direction andN samples in a horizontal direction, where N represents a nonnegativeinteger value. The samples in a CU may be arranged in rows and columns.Moreover, CUs need not necessarily have the same number of samples inthe horizontal direction as in the vertical direction. For example, CUsmay comprise N×M samples, where M is not necessarily equal to N.

Video encoder 200 encodes video data for CUs representing predictionand/or residual information, and other information. The predictioninformation indicates how the CU is to be predicted in order to form aprediction block for the CU. The residual information generallyrepresents sample-by-sample differences between samples of the CU priorto encoding and the prediction block.

To predict a CU, video encoder 200 may generally form a prediction blockfor the CU through inter-prediction or intra-prediction.Inter-prediction generally refers to predicting the CU from data of apreviously coded picture, whereas intra-prediction generally refers topredicting the CU from previously coded data of the same picture. Toperform inter-prediction, video encoder 200 may generate the predictionblock using one or more motion vectors. Video encoder 200 may generallyperform a motion search to identify a reference block that closelymatches the CU, e.g., in terms of differences between the CU and thereference block. Video encoder 200 may calculate a difference metricusing a sum of absolute difference (SAD), sum of squared differences(SSD), mean absolute difference (MAD), mean squared differences (MSD),or other such difference calculations to determine whether a referenceblock closely matches the current CU. In some examples, video encoder200 may predict the current CU using uni-directional prediction orbi-directional prediction.

Some examples of VVC also provide an affine motion compensation mode,which may be considered an inter-prediction mode. In affine motioncompensation mode, video encoder 200 may determine two or more motionvectors that represent non-translational motion, such as zoom in or out,rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder 200 may select anintra-prediction mode to generate the prediction block. Some examples ofVVC provide sixty-seven intra-prediction modes, including variousdirectional modes, as well as planar mode and DC mode. In general, videoencoder 200 selects an intra-prediction mode that describes neighboringsamples to a current block (e.g., a block of a CU) from which to predictsamples of the current block. Such samples may generally be above, aboveand to the left, or to the left of the current block in the same pictureas the current block, assuming video encoder 200 codes CTUs and CUs inraster scan order (left to right, top to bottom).

Video encoder 200 encodes data representing the prediction mode for acurrent block. For example, for inter-prediction modes, video encoder200 may encode data representing which of the various availableinter-prediction modes is used, as well as motion information for thecorresponding mode. For uni-directional or bi-directionalinter-prediction, for example, video encoder 200 may encode motionvectors using advanced motion vector prediction (AMVP) or merge mode.Video encoder 200 may use similar modes to encode motion vectors foraffine motion compensation mode.

Following prediction, such as intra-prediction or inter-prediction of ablock, video encoder 200 may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. Video encoder 200 mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, video encoder 200 may apply a discrete cosine transform (DCT),an integer transform, a wavelet transform, or a conceptually similartransform to residual video data. Additionally, video encoder 200 mayapply a secondary transform following the first transform, such as amode-dependent non-separable secondary transform (MDNSST), a signaldependent transform, a Karhunen-Loeve transform (KLT), or the like.Video encoder 200 produces transform coefficients following applicationof the one or more transforms.

As noted above, following any transforms to produce transformcoefficients, video encoder 200 may perform quantization of thetransform coefficients. Quantization generally refers to a process inwhich transform coefficients are quantized to possibly reduce the amountof data used to represent the transform coefficients, providing furthercompression. By performing the quantization process, video encoder 200may reduce the bit depth associated with some or all of the transformcoefficients. For example, video encoder 200 may round an n-bit valuedown to an m-bit value during quantization, where n is greater than m.In some examples, to perform quantization, video encoder 200 may performa bitwise right-shift of the value to be quantized.

Following quantization, video encoder 200 may scan the transformcoefficients, producing a one-dimensional vector from thetwo-dimensional matrix including the quantized transform coefficients.The scan may be designed to place higher energy (and therefore lowerfrequency) transform coefficients at the front of the vector and toplace lower energy (and therefore higher frequency) transformcoefficients at the back of the vector. In some examples, video encoder200 may utilize a predefined scan order to scan the quantized transformcoefficients to produce a serialized vector, and then entropy encode thequantized transform coefficients of the vector. In other examples, videoencoder 200 may perform an adaptive scan. After scanning the quantizedtransform coefficients to form the one-dimensional vector, video encoder200 may entropy encode the one-dimensional vector, e.g., according tocontext-adaptive binary arithmetic coding (CABAC). Video encoder 200 mayalso entropy encode values for syntax elements describing metadataassociated with the encoded video data for use by video decoder 300 indecoding the video data.

To perform CABAC, video encoder 200 may assign a context within acontext model to a symbol to be transmitted. The context may relate to,for example, whether neighboring values of the symbol are zero-valued ornot. The probability determination may be based on a context assigned tothe symbol.

Video encoder 200 may further generate syntax data, such as block-basedsyntax data, picture-based syntax data, and sequence-based syntax data,to video decoder 300, e.g., in a picture header, a block header, a sliceheader, or other syntax data, such as a sequence parameter set (SPS),picture parameter set (PPS), or video parameter set (VPS). Video decoder300 may likewise decode such syntax data to determine how to decodecorresponding video data.

In this manner, video encoder 200 may generate a bitstream includingencoded video data, e.g., syntax elements describing partitioning of apicture into blocks (e.g., CUs) and prediction and/or residualinformation for the blocks. Ultimately, video decoder 300 may receivethe bitstream and decode the encoded video data.

In general, video decoder 300 performs a reciprocal process to thatperformed by video encoder 200 to decode the encoded video data of thebitstream. For example, video decoder 300 may decode values for syntaxelements of the bitstream using CABAC in a manner substantially similarto, albeit reciprocal to, the CABAC encoding process of video encoder200. The syntax elements may define partitioning information forpartitioning of a picture into CTUs, and partitioning of each CTUaccording to a corresponding partition structure, such as a QTBTstructure, to define CUs of the CTU. The syntax elements may furtherdefine prediction and residual information for blocks (e.g., CUs) ofvideo data.

The residual information may be represented by, for example, quantizedtransform coefficients. Video decoder 300 may inverse quantize andinverse transform the quantized transform coefficients of a block toreproduce a residual block for the block. Video decoder 300 uses asignaled prediction mode (intra- or inter-prediction) and relatedprediction information (e.g., motion information for inter-prediction)to form a prediction block for the block. Video decoder 300 may thencombine the prediction block and the residual block (on asample-by-sample basis) to reproduce the original block. Video decoder300 may perform additional processing, such as performing a deblockingprocess to reduce visual artifacts along boundaries of the block.

In accordance with the techniques of this disclosure, as will beexplained in more detail below, video encoder 200 may be configured toencode a block of the video data, reconstruct the block of the videodata, apply an adaptive loop filter to the reconstructed block of thevideo data to create a filtered block of the video data, and signal anadaptation parameter sets (APS) in an encoded video bitstream for theblock of the video data, wherein the APS includes a plurality ofadaptive loop filter sets, including the applied adaptive loop filter,for luma components of the block of the video data. In a reciprocalfashion, video decoder 300 may be configured to decode a block of thevideo data, receive an adaptation parameter sets (APS) in an encodedvideo bitstream for the block of the video data, wherein the APSincludes a plurality of adaptive loop filter sets for luma components ofthe block of the video data, determine an adaptive loop filter from theplurality of adaptive loop filter sets in the APS to apply to thedecoded block of the video data, and apply the determined adaptive loopfilter to the decoded block of the video data to create a filtered blockof the video data.

This disclosure may generally refer to “signaling” certain information,such as syntax elements. The term “signaling” may generally refer to thecommunication of values syntax elements and/or other data used to decodeencoded video data. That is, video encoder 200 may signal values forsyntax elements in the bitstream. In general, signaling refers togenerating a value in the bitstream. As noted above, source device 102may transport the bitstream to destination device 116 substantially inreal time, or not in real time, such as might occur when storing syntaxelements to storage device 112 for later retrieval by destination device116.

FIGS. 2A and 2B are conceptual diagram illustrating an example quadtreebinary tree (QTBT) structure 130, and a corresponding coding tree unit(CTU) 132. The solid lines represent quadtree splitting, and dottedlines indicate binary tree splitting. In each split (i.e., non-leaf)node of the binary tree, one flag is signaled to indicate whichsplitting type (i.e., horizontal or vertical) is used, where 0 indicateshorizontal splitting and 1 indicates vertical splitting in this example.For the quadtree splitting, there is no need to indicate the splittingtype, since quadtree nodes split a block horizontally and verticallyinto 4 sub-blocks with equal size. Accordingly, video encoder 200 mayencode, and video decoder 300 may decode, syntax elements (such assplitting information) for a region tree level of QTBT structure 130(i.e., the solid lines) and syntax elements (such as splittinginformation) for a prediction tree level of QTBT structure 130 (i.e.,the dashed lines). Video encoder 200 may encode, and video decoder 300may decode, video data, such as prediction and transform data, for CUsrepresented by terminal leaf nodes of QTBT structure 130.

In general, CTU 132 of FIG. 2B may be associated with parametersdefining sizes of blocks corresponding to nodes of QTBT structure 130 atthe first and second levels. These parameters may include a CTU size(representing a size of CTU 132 in samples), a minimum quadtree size(MinQTSize, representing a minimum allowed quadtree leaf node size), amaximum binary tree size (MaxBTSize, representing a maximum allowedbinary tree root node size), a maximum binary tree depth (MaxBTDepth,representing a maximum allowed binary tree depth), and a minimum binarytree size (MinBTSize, representing the minimum allowed binary tree leafnode size).

The root node of a QTBT structure corresponding to a CTU may have fourchild nodes at the first level of the QTBT structure, each of which maybe partitioned according to quadtree partitioning. That is, nodes of thefirst level are either leaf nodes (having no child nodes) or have fourchild nodes. The example of QTBT structure 130 represents such nodes asincluding the parent node and child nodes having solid lines forbranches. If nodes of the first level are not larger than the maximumallowed binary tree root node size (MaxBTSize), they can be furtherpartitioned by respective binary trees. The binary tree splitting of onenode can be iterated until the nodes resulting from the split reach theminimum allowed binary tree leaf node size (MinBTSize) or the maximumallowed binary tree depth (MaxBTDepth). The example of QTBT structure130 represents such nodes as having dashed lines for branches. Thebinary tree leaf node is referred to as a coding unit (CU), which isused for prediction (e.g., intra-picture or inter-picture prediction)and transform, without any further partitioning. As discussed above, CUsmay also be referred to as “video blocks” or “blocks.”

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 (luma samples and two corresponding 64×64 chroma samples),the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, theMinBTSize (for both width and height) is set as 4, and the MaxBTDepth isset as 4. The quadtree partitioning is applied to the CTU first togenerate quad-tree leaf nodes. The quadtree leaf nodes may have a sizefrom 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If theleaf quadtree node is 128×128, it will not be further split by thebinary tree, since the size exceeds the MaxBTSize (i.e., 64×64, in thisexample). Otherwise, the leaf quadtree node will be further partitionedby the binary tree. Therefore, the quadtree leaf node is also the rootnode for the binary tree and has the binary tree depth as 0. When thebinary tree depth reaches MaxBTDepth (4, in this example), no furthersplitting is permitted. When the binary tree node has width equal toMinBTSize (4, in this example), it implies no further horizontalsplitting is permitted. Similarly, a binary tree node having a heightequal to MinBTSize implies no further vertical splitting is permittedfor that binary tree node. As noted above, leaf nodes of the binary treeare referred to as CUs, and are further processed according toprediction and transform without further partitioning.

FIG. 3 is a block diagram illustrating an example video encoder 200 thatmay perform the techniques of this disclosure. FIG. 3 is provided forpurposes of explanation and should not be considered limiting of thetechniques as broadly exemplified and described in this disclosure. Forpurposes of explanation, this disclosure describes video encoder 200 inthe context of video coding standards such as the HEVC video codingstandard and the H.266 video coding standard in development. However,the techniques of this disclosure are not limited to these video codingstandards, and are applicable generally to video encoding and decoding.

In the example of FIG. 3, video encoder 200 includes video data memory230, mode selection unit 202, residual generation unit 204, transformprocessing unit 206, quantization unit 208, inverse quantization unit210, inverse transform processing unit 212, reconstruction unit 214,filter unit 216, decoded picture buffer (DPB) 218, and entropy encodingunit 220. Any or all of video data memory 230, mode selection unit 202,residual generation unit 204, transform processing unit 206,quantization unit 208, inverse quantization unit 210, inverse transformprocessing unit 212, reconstruction unit 214, filter unit 216, DPB 218,and entropy encoding unit 220 may be implemented in one or moreprocessors or in processing circuitry. Moreover, video encoder 200 mayinclude additional or alternative processors or processing circuitry toperform these and other functions.

Video data memory 230 may store video data to be encoded by thecomponents of video encoder 200. Video encoder 200 may receive the videodata stored in video data memory 230 from, for example, video source 104(FIG. 1). DPB 218 may act as a reference picture memory that storesreference video data for use in prediction of subsequent video data byvideo encoder 200. Video data memory 230 and DPB 218 may be formed byany of a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. Video datamemory 230 and DPB 218 may be provided by the same memory device orseparate memory devices. In various examples, video data memory 230 maybe on-chip with other components of video encoder 200, as illustrated,or off-chip relative to those components.

In this disclosure, reference to video data memory 230 should not beinterpreted as being limited to memory internal to video encoder 200,unless specifically described as such, or memory external to videoencoder 200, unless specifically described as such. Rather, reference tovideo data memory 230 should be understood as reference memory thatstores video data that video encoder 200 receives for encoding (e.g.,video data for a current block that is to be encoded). Memory 106 ofFIG. 1 may also provide temporary storage of outputs from the variousunits of video encoder 200.

The various units of FIG. 3 are illustrated to assist with understandingthe operations performed by video encoder 200. The units may beimplemented as fixed-function circuits, programmable circuits, or acombination thereof. Fixed-function circuits refer to circuits thatprovide particular functionality, and are preset on the operations thatcan be performed. Programmable circuits refer to circuits that canprogrammed to perform various tasks, and provide flexible functionalityin the operations that can be performed. For instance, programmablecircuits may execute software or firmware that cause the programmablecircuits to operate in the manner defined by instructions of thesoftware or firmware. Fixed-function circuits may execute softwareinstructions (e.g., to receive parameters or output parameters), but thetypes of operations that the fixed-function circuits perform aregenerally immutable. In some examples, the one or more of the units maybe distinct circuit blocks (fixed-function or programmable), and in someexamples, the one or more units may be integrated circuits.

Video encoder 200 may include arithmetic logic units (ALUs), elementaryfunction units (EFUs), digital circuits, analog circuits, and/orprogrammable cores, formed from programmable circuits. In examples wherethe operations of video encoder 200 are performed using softwareexecuted by the programmable circuits, memory 106 (FIG. 1) may store theobject code of the software that video encoder 200 receives andexecutes, or another memory within video encoder 200 (not shown) maystore such instructions.

Video data memory 230 is configured to store received video data. Videoencoder 200 may retrieve a picture of the video data from video datamemory 230 and provide the video data to residual generation unit 204and mode selection unit 202. Video data in video data memory 230 may beraw video data that is to be encoded.

Mode selection unit 202 includes a motion estimation unit 222, motioncompensation unit 224, and an intra-prediction unit 226. Mode selectionunit 202 may include additional functional units to perform videoprediction in accordance with other prediction modes. As examples, modeselection unit 202 may include a palette unit, an intra-block copy unit(which may be part of motion estimation unit 222 and/or motioncompensation unit 224), an affine unit, a linear model (LM) unit, or thelike.

Mode selection unit 202 generally coordinates multiple encoding passesto test combinations of encoding parameters and resultingrate-distortion values for such combinations. The encoding parametersmay include partitioning of CTUs into CUs, prediction modes for the CUs,transform types for residual data of the CUs, quantization parametersfor residual data of the CUs, and so on. Mode selection unit 202 mayultimately select the combination of encoding parameters havingrate-distortion values that are better than the other testedcombinations.

Video encoder 200 may partition a picture retrieved from video datamemory 230 into a series of CTUs, and encapsulate one or more CTUswithin a slice. Mode selection unit 202 may partition a CTU of thepicture in accordance with a tree structure, such as the QTBT structureor the quad-tree structure of HEVC described above. As described above,video encoder 200 may form one or more CUs from partitioning a CTUaccording to the tree structure. Such a CU may also be referred togenerally as a “video block” or “block.”

In general, mode selection unit 202 also controls the components thereof(e.g., motion estimation unit 222, motion compensation unit 224, andintra-prediction unit 226) to generate a prediction block for a currentblock (e.g., a current CU, or in HEVC, the overlapping portion of a PUand a TU). For inter-prediction of a current block, motion estimationunit 222 may perform a motion search to identify one or more closelymatching reference blocks in one or more reference pictures (e.g., oneor more previously coded pictures stored in DPB 218). In particular,motion estimation unit 222 may calculate a value representative of howsimilar a potential reference block is to the current block, e.g.,according to sum of absolute difference (SAD), sum of squareddifferences (SSD), mean absolute difference (MAD), mean squareddifferences (MSD), or the like. Motion estimation unit 222 may generallyperform these calculations using sample-by-sample differences betweenthe current block and the reference block being considered. Motionestimation unit 222 may identify a reference block having a lowest valueresulting from these calculations, indicating a reference block thatmost closely matches the current block.

Motion estimation unit 222 may form one or more motion vectors (MVs)that defines the positions of the reference blocks in the referencepictures relative to the position of the current block in a currentpicture. Motion estimation unit 222 may then provide the motion vectorsto motion compensation unit 224. For example, for uni-directionalinter-prediction, motion estimation unit 222 may provide a single motionvector, whereas for bi-directional inter-prediction, motion estimationunit 222 may provide two motion vectors. Motion compensation unit 224may then generate a prediction block using the motion vectors. Forexample, motion compensation unit 224 may retrieve data of the referenceblock using the motion vector. As another example, if the motion vectorhas fractional sample precision, motion compensation unit 224 mayinterpolate values for the prediction block according to one or moreinterpolation filters. Moreover, for bi-directional inter-prediction,motion compensation unit 224 may retrieve data for two reference blocksidentified by respective motion vectors and combine the retrieved data,e.g., through sample-by-sample averaging or weighted averaging.

As another example, for intra-prediction, or intra-prediction coding,intra-prediction unit 226 may generate the prediction block from samplesneighboring the current block. For example, for directional modes,intra-prediction unit 226 may generally mathematically combine values ofneighboring samples and populate these calculated values in the defineddirection across the current block to produce the prediction block. Asanother example, for DC mode, intra-prediction unit 226 may calculate anaverage of the neighboring samples to the current block and generate theprediction block to include this resulting average for each sample ofthe prediction block.

Mode selection unit 202 provides the prediction block to residualgeneration unit 204. Residual generation unit 204 receives a raw,unencoded version of the current block from video data memory 230 andthe prediction block from mode selection unit 202. Residual generationunit 204 calculates sample-by-sample differences between the currentblock and the prediction block. The resulting sample-by-sampledifferences define a residual block for the current block. In someexamples, residual generation unit 204 may also determine differencesbetween sample values in the residual block to generate a residual blockusing residual differential pulse code modulation (RDPCM). In someexamples, residual generation unit 204 may be formed using one or moresubtractor circuits that perform binary subtraction.

In examples where mode selection unit 202 partitions CUs into PUs, eachPU may be associated with a luma prediction unit and correspondingchroma prediction units. Video encoder 200 and video decoder 300 maysupport PUs having various sizes. As indicated above, the size of a CUmay refer to the size of the luma coding block of the CU and the size ofa PU may refer to the size of a luma prediction unit of the PU. Assumingthat the size of a particular CU is 2N×2N, video encoder 200 may supportPU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder200 and video decoder 300 may also support asymmetric partitioning forPU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N for inter prediction.

In examples where mode selection unit does not further partition a CUinto PUs, each CU may be associated with a luma coding block andcorresponding chroma coding blocks. As above, the size of a CU may referto the size of the luma coding block of the CU. The video encoder 200and video decoder 300 may support CU sizes of 2N×2N, 2N×N, or N×2N.

For other video coding techniques such as an intra-block copy modecoding, an affine-mode coding, and linear model (LM) mode coding, as fewexamples, mode selection unit 202, via respective units associated withthe coding techniques, generates a prediction block for the currentblock being encoded. In some examples, such as palette mode coding, modeselection unit 202 may not generate a prediction block, and insteadgenerate syntax elements that indicate the manner in which toreconstruct the block based on a selected palette. In such modes, modeselection unit 202 may provide these syntax elements to entropy encodingunit 220 to be encoded.

As described above, residual generation unit 204 receives the video datafor the current block and the corresponding prediction block. Residualgeneration unit 204 then generates a residual block for the currentblock. To generate the residual block, residual generation unit 204calculates sample-by-sample differences between the prediction block andthe current block.

Transform processing unit 206 applies one or more transforms to theresidual block to generate a block of transform coefficients (referredto herein as a “transform coefficient block”). Transform processing unit206 may apply various transforms to a residual block to form thetransform coefficient block. For example, transform processing unit 206may apply a discrete cosine transform (DCT), a directional transform, aKarhunen-Loeve transform (KLT), or a conceptually similar transform to aresidual block. In some examples, transform processing unit 206 mayperform multiple transforms to a residual block, e.g., a primarytransform and a secondary transform, such as a rotational transform. Insome examples, transform processing unit 206 does not apply transformsto a residual block.

Quantization unit 208 may quantize the transform coefficients in atransform coefficient block, to produce a quantized transformcoefficient block. Quantization unit 208 may quantize transformcoefficients of a transform coefficient block according to aquantization parameter (QP) value associated with the current block.Video encoder 200 (e.g., via mode selection unit 202) may adjust thedegree of quantization applied to the coefficient blocks associated withthe current block by adjusting the QP value associated with the CU.Quantization may introduce loss of information, and thus, quantizedtransform coefficients may have lower precision than the originaltransform coefficients produced by transform processing unit 206.

Inverse quantization unit 210 and inverse transform processing unit 212may apply inverse quantization and inverse transforms to a quantizedtransform coefficient block, respectively, to reconstruct a residualblock from the transform coefficient block. Reconstruction unit 214 mayproduce a reconstructed block corresponding to the current block (albeitpotentially with some degree of distortion) based on the reconstructedresidual block and a prediction block generated by mode selection unit202. For example, reconstruction unit 214 may add samples of thereconstructed residual block to corresponding samples from theprediction block generated by mode selection unit 202 to produce thereconstructed block.

Filter unit 216 may perform one or more filter operations onreconstructed blocks. For example, filter unit 216 may performdeblocking operations to reduce blockiness artifacts along edges of CUs.Filter unit 216 may also perform one or more of the techniques of thisdisclosure for determining and applying adaptive loop filters, as willbe described in more detail below. Operations of filter unit 216 may beskipped, in some examples.

In accordance with the techniques of this disclosure, as will beexplained in more detail below, filter unit 216 may be configured toperform one or more techniques of the disclosure. For example, videoencoder 200 may be configured to encode a block of the video data, andreconstruct the block of the video data using any of the predictiontechniques described above or other encoding techniques. Filter unit 216may be configured to apply an adaptive loop filter to the reconstructedblock of the video data to create a filtered block of the video data,and signal an adaptation parameter sets (APS) in an encoded videobitstream for the block of the video data, wherein the APS includes aplurality of adaptive loop filter sets, including the applied adaptiveloop filter, for luma components of the block of the video data.

Video encoder 200 stores reconstructed blocks in DPB 218. For instance,in examples where operations of filter unit 216 are not needed,reconstruction unit 214 may store reconstructed blocks to DPB 218. Inexamples where operations of filter unit 216 are needed, filter unit 216may store the filtered reconstructed blocks to DPB 218. Motionestimation unit 222 and motion compensation unit 224 may retrieve areference picture from DPB 218, formed from the reconstructed (andpotentially filtered) blocks, to inter-predict blocks of subsequentlyencoded pictures. In addition, intra-prediction unit 226 may usereconstructed blocks in DPB 218 of a current picture to intra-predictother blocks in the current picture.

In general, entropy encoding unit 220 may entropy encode syntax elementsreceived from other functional components of video encoder 200. Forexample, entropy encoding unit 220 may entropy encode quantizedtransform coefficient blocks from quantization unit 208. As anotherexample, entropy encoding unit 220 may entropy encode prediction syntaxelements (e.g., motion information for inter-prediction or intra-modeinformation for intra-prediction) from mode selection unit 202. Entropyencoding unit 220 may perform one or more entropy encoding operations onthe syntax elements, which are another example of video data, togenerate entropy-encoded data. For example, entropy encoding unit 220may perform a context-adaptive variable length coding (CAVLC) operation,a CABAC operation, a variable-to-variable (V2V) length coding operation,a syntax-based context-adaptive binary arithmetic coding (SBAC)operation, a Probability Interval Partitioning Entropy (PIPE) codingoperation, an Exponential-Golomb encoding operation, or another type ofentropy encoding operation on the data. In some examples, entropyencoding unit 220 may operate in bypass mode where syntax elements arenot entropy encoded.

Video encoder 200 may output a bitstream that includes the entropyencoded syntax elements needed to reconstruct blocks of a slice orpicture. In particular, entropy encoding unit 220 may output thebitstream.

The operations described above are described with respect to a block.Such description should be understood as being operations for a lumacoding block and/or chroma coding blocks. As described above, in someexamples, the luma coding block and chroma coding blocks are luma andchroma components of a CU. In some examples, the luma coding block andthe chroma coding blocks are luma and chroma components of a PU.

In some examples, operations performed with respect to a luma codingblock need not be repeated for the chroma coding blocks. As one example,operations to identify a motion vector (MV) and reference picture for aluma coding block need not be repeated for identifying a MV andreference picture for the chroma blocks. Rather, the MV for the lumacoding block may be scaled to determine the MV for the chroma blocks,and the reference picture may be the same. As another example, theintra-prediction process may be the same for the luma coding blocks andthe chroma coding blocks.

Video encoder 200 represents an example of a device configured to encodea block of the video data, reconstruct the block of the video data,apply an adaptive loop filter to the reconstructed block of the videodata to create a filtered block of the video data, and signal anadaptation parameter sets (APS) in an encoded video bitstream for theblock of the video data, wherein the APS includes a plurality ofadaptive loop filter sets, including the applied adaptive loop filter,for luma components of the block of the video data.

FIG. 4 is a block diagram illustrating an example video decoder 300 thatmay perform the techniques of this disclosure. FIG. 4 is provided forpurposes of explanation and is not limiting on the techniques as broadlyexemplified and described in this disclosure. For purposes ofexplanation, this disclosure describes video decoder 300 is describedaccording to the techniques of JEM, VVC, and HEVC. However, thetechniques of this disclosure may be performed by video coding devicesthat are configured to other video coding standards.

In the example of FIG. 4, video decoder 300 includes coded picturebuffer (CPB) memory 320, entropy decoding unit 302, predictionprocessing unit 304, inverse quantization unit 306, inverse transformprocessing unit 308, reconstruction unit 310, filter unit 312, anddecoded picture buffer (DPB) 314. Any or all of CPB memory 320, entropydecoding unit 302, prediction processing unit 304, inverse quantizationunit 306, inverse transform processing unit 308, reconstruction unit310, filter unit 312, and DPB 314 may be implemented in one or moreprocessors or in processing circuitry. Moreover, video decoder 300 mayinclude additional or alternative processors or processing circuitry toperform these and other functions.

Prediction processing unit 304 includes motion compensation unit 316 andintra-prediction unit 318. Prediction processing unit 304 may includeaddition units to perform prediction in accordance with other predictionmodes. As examples, prediction processing unit 304 may include a paletteunit, an intra-block copy unit (which may form part of motioncompensation unit 316), an affine unit, a linear model (LM) unit, or thelike. In other examples, video decoder 300 may include more, fewer, ordifferent functional components.

CPB memory 320 may store video data, such as an encoded video bitstream,to be decoded by the components of video decoder 300. The video datastored in CPB memory 320 may be obtained, for example, fromcomputer-readable medium 110 (FIG. 1). CPB memory 320 may include a CPBthat stores encoded video data (e.g., syntax elements) from an encodedvideo bitstream. Also, CPB memory 320 may store video data other thansyntax elements of a coded picture, such as temporary data representingoutputs from the various units of video decoder 300. DPB 314 generallystores decoded pictures, which video decoder 300 may output and/or useas reference video data when decoding subsequent data or pictures of theencoded video bitstream. CPB memory 320 and DPB 314 may be formed by anyof a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. CPB memory 320and DPB 314 may be provided by the same memory device or separate memorydevices. In various examples, CPB memory 320 may be on-chip with othercomponents of video decoder 300, or off-chip relative to thosecomponents.

Additionally or alternatively, in some examples, video decoder 300 mayretrieve coded video data from memory 120 (FIG. 1). That is, memory 120may store data as discussed above with CPB memory 320. Likewise, memory120 may store instructions to be executed by video decoder 300, whensome or all of the functionality of video decoder 300 is implemented insoftware to executed by processing circuitry of video decoder 300.

The various units shown in FIG. 4 are illustrated to assist withunderstanding the operations performed by video decoder 300. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Similar to FIG. 3, fixed-function circuits referto circuits that provide particular functionality, and are preset on theoperations that can be performed. Programmable circuits refer tocircuits that can programmed to perform various tasks, and provideflexible functionality in the operations that can be performed. Forinstance, programmable circuits may execute software or firmware thatcause the programmable circuits to operate in the manner defined byinstructions of the software or firmware. Fixed-function circuits mayexecute software instructions (e.g., to receive parameters or outputparameters), but the types of operations that the fixed-functioncircuits perform are generally immutable. In some examples, the one ormore of the units may be distinct circuit blocks (fixed-function orprogrammable), and in some examples, the one or more units may beintegrated circuits.

Video decoder 300 may include ALUs, EFUs, digital circuits, analogcircuits, and/or programmable cores formed from programmable circuits.In examples where the operations of video decoder 300 are performed bysoftware executing on the programmable circuits, on-chip or off-chipmemory may store instructions (e.g., object code) of the software thatvideo decoder 300 receives and executes.

Entropy decoding unit 302 may receive encoded video data from the CPBand entropy decode the video data to reproduce syntax elements.Prediction processing unit 304, inverse quantization unit 306, inversetransform processing unit 308, reconstruction unit 310, and filter unit312 may generate decoded video data based on the syntax elementsextracted from the bitstream.

In general, video decoder 300 reconstructs a picture on a block-by-blockbasis. Video decoder 300 may perform a reconstruction operation on eachblock individually (where the block currently being reconstructed, i.e.,decoded, may be referred to as a “current block”).

Entropy decoding unit 302 may entropy decode syntax elements definingquantized transform coefficients of a quantized transform coefficientblock, as well as transform information, such as a quantizationparameter (QP) and/or transform mode indication(s). Inverse quantizationunit 306 may use the QP associated with the quantized transformcoefficient block to determine a degree of quantization and, likewise, adegree of inverse quantization for inverse quantization unit 306 toapply. Inverse quantization unit 306 may, for example, perform a bitwiseleft-shift operation to inverse quantize the quantized transformcoefficients. Inverse quantization unit 306 may thereby form a transformcoefficient block including transform coefficients.

After inverse quantization unit 306 forms the transform coefficientblock, inverse transform processing unit 308 may apply one or moreinverse transforms to the transform coefficient block to generate aresidual block associated with the current block. For example, inversetransform processing unit 308 may apply an inverse DCT, an inverseinteger transform, an inverse Karhunen-Loeve transform (KLT), an inverserotational transform, an inverse directional transform, or anotherinverse transform to the coefficient block.

Furthermore, prediction processing unit 304 generates a prediction blockaccording to prediction information syntax elements that were entropydecoded by entropy decoding unit 302. For example, if the predictioninformation syntax elements indicate that the current block isinter-predicted, motion compensation unit 316 may generate theprediction block. In this case, the prediction information syntaxelements may indicate a reference picture in DPB 314 from which toretrieve a reference block, as well as a motion vector identifying alocation of the reference block in the reference picture relative to thelocation of the current block in the current picture. Motioncompensation unit 316 may generally perform the inter-prediction processin a manner that is substantially similar to that described with respectto motion compensation unit 224 (FIG. 3).

As another example, if the prediction information syntax elementsindicate that the current block is intra-predicted, intra-predictionunit 318 may generate the prediction block according to anintra-prediction mode indicated by the prediction information syntaxelements. Again, intra-prediction unit 318 may generally perform theintra-prediction process in a manner that is substantially similar tothat described with respect to intra-prediction unit 226 (FIG. 3).Intra-prediction unit 318 may retrieve data of neighboring samples tothe current block from DPB 314.

Reconstruction unit 310 may reconstruct the current block using theprediction block and the residual block. For example, reconstructionunit 310 may add samples of the residual block to corresponding samplesof the prediction block to reconstruct the current block.

Filter unit 312 may perform one or more filter operations onreconstructed blocks. For example, filter unit 312 may performdeblocking operations to reduce blockiness artifacts along edges of thereconstructed blocks. Filter unit 312 may also perform one or more ofthe techniques of this disclosure for determining and applying adaptiveloop filters, as will be described in more detail below. Operations offilter unit 312 are not necessarily performed in all examples.

As will be explained in more detail below, filter unit 312 may beconfigured to perform the techniques of this disclosure. For example,video decoder 300 may be configured to decode a block of the video data,and receive an adaptation parameter sets (APS) in an encoded videobitstream for the block of the video data, wherein the APS includes aplurality of adaptive loop filter sets for luma components of the blockof the video data. Filter unit 312 may be configured to determine anadaptive loop filter from the plurality of adaptive loop filter sets inthe APS to apply to the decoded block of the video data, and apply thedetermined adaptive loop filter to the decoded block of the video datato create a filtered block of the video data.

Video decoder 300 may store the reconstructed blocks in DPB 314. Asdiscussed above, DPB 314 may provide reference information, such assamples of a current picture for intra-prediction and previously decodedpictures for subsequent motion compensation, to prediction processingunit 304. Moreover, video decoder 300 may output decoded pictures fromDPB for subsequent presentation on a display device, such as displaydevice 118 of FIG. 1.

In this manner, video decoder 300 represents an example of a videodecoding device including a memory configured to store video data, andone or more processing units implemented in circuitry and configured todecode a block of the video data, receive an adaptation parameter sets(APS) in an encoded video bitstream for the block of the video data,wherein the APS includes a plurality of adaptive loop filter sets forluma components of the block of the video data, determine an adaptiveloop filter from the plurality of adaptive loop filter sets in the APSto apply to the decoded block of the video data, and apply thedetermined adaptive loop filter to the decoded block of the video datato create a filtered block of the video data.

As described above, the example techniques described in this disclosureare related to adaptive loop filters. The following provides additionalinformation regarding adaptive loop filtering.

Adaptive Loop Filters in VVC Test Model 5.0 (VTM-5.0)

In the field of video coding, it is common to apply filtering in orderto enhance the quality of a decoded/reconstructed video signal. Thefilter can be applied as a post-filter, where a filtered frame is notused for prediction of future frames. In other examples, the filter canbe applied as an in-loop filter, where the filtered frame is used topredict future frame (e.g., filter unit 216 and filter unit 312 in FIGS.3 and 4, respectively). Accordingly, even in examples where videoencoder 200 is not configured to display the picture, video encoder 200may be configured to perform the adaptive loop filter techniques (e.g.,via filter unit 216) as part of the in-loop filter. Video decoder 300may be configured to perform the adaptive loop filter techniques (e.g.,via filter unit 312) for in-loop filtering, or filter unit 312 may be onthe output of DPB 314 as a post-filter (or some combination of in-loopand post-filter). A filter can be designed, for example, by minimizingthe error between the original signal and the decoded filtered signal.In this disclosure, a reconstructed block or picture may refer to outputof reconstruction unit 214 of video encoder 200. A decoded block orpicture may refer to the output of reconstruction unit 310 of videodecoder 300.

Adaptive Loop Filter with Clipping

In VTM-5.0, the decoded filter coefficients f (k,l) and clipping valuesc(k,l) are applied to the decoded/reconstructed image R(i,j) as follows:

{tilde over (R)}(i,j)=R(i,j)+Σ_(k,l=(−K,−K),k,l≠(0,0)) ^(K,K)f(k,l)*clip3(−c(k,l),c(k,l),R(i+k,j+l)−R(i,j))  (1)

In some examples, video encoder 200 and video decoder 300 may beconfigured to apply a 7×7 filter to luma components of a block and applya 5×5 filter to chroma components of a block. FIG. 5 is a conceptualdiagram illustrating example adaptive loop filter supports. As shown inFIG. 5, adaptive loop filter support 500 is a 5×5 diamond filter supportand adaptive loop filter support 502 is a 7×7 diamond filter support,where the C# values are filter coefficients.

Pixel Classification

For the luma component of a block, video encoder 200 and video decoder300 may be configured to classify 4×4 blocks in the whole picture basedon a 1D Laplacian direction (up to 5 directions) and a 2D Laplacianactivity (up to 5 activity values). Video encoder 200 and video decoder300 may be configured to further quantize the calculation of directionDir_(b) and unquanitzed activity Act_(b). As one example, Act_(b) isquantized to the range of 0 to 4, inclusively.

First, video encoder 200 and video decoder 300 may be configured tocalculate values of two diagonal gradients, in addition to thehorizontal and vertical gradients used in the existing adaptive loopfilter (e.g., in VVC Draft 5), using a 1-D Laplacian. As can be seenfrom equations (2) to (5) below, video encoder 200 and video decoder 300may be configured to uses the sum of gradients of all pixels within an8×8 window that covers a target pixel as the represented gradient oftarget pixel, where R(k,l) is the reconstructed pixels at location (k,l)and indices i and j refer to the coordinates of the upper left pixel inthe 4×4 block. Each pixel is associated with four gradient values, withvertical gradient denoted by g_(v), horizontal gradient denoted byg_(h), 135 degree diagonal gradient denoted by g_(d1), and 45 degreediagonal gradient denoted by g_(d2).

$\begin{matrix}{{g_{v} = {\sum\limits_{k = {i - 2}}^{i + 5}{\sum\limits_{l = {j - 2}}^{j + 3}V_{k,l}}}},} & (2)\end{matrix}$

V_(k,l)=|2R(k,l)−R(k,l−1)−R(k,l+1)|, if k and l are both even numbers,or both k and l are not even numbers. Otherwise, 0.

$\begin{matrix}{{g_{h} = {\sum\limits_{k = {i - 2}}^{i + 5}{\sum\limits_{l = {j - 2}}^{j + 5}H_{k,l}}}},} & (3)\end{matrix}$

H_(k,l)=|2R(k,l)−R(k−1,l)+R(k+1,l)|, if k and l are both even numbness,or both k and l are not even numbers. Otherwise, 0.

$\begin{matrix}{{g_{d\; 1} = {\sum\limits_{k = {i - 2}}^{i + 5}{\sum\limits_{l = {j - 2}}^{j + 5}{D\; 1_{k,l}}}}},} & (4)\end{matrix}$

D1_(k,l)=|2R (k,l)−R(k−1,l−1)−R(k+1,l−1)|, if k and l are both evennumbers, or both k and 1 are not even numbers. Otherwise, 0.

$\begin{matrix}{{g_{d\; 2} = {\sum\limits_{k = {i - 2}}^{i + 5}{\sum\limits_{l = {j - 2}}^{j + 5}{D\; 2_{k,l}}}}},} & (5)\end{matrix}$

D2_(k,l)=|2R(k,l)−R(k−1,l+1)−R(k+1,l−1)|, if k and l are both evennumbers, or both k and l are not even numbers. Otherwise, 0.

To assign the directionality Dir_(b), video encoder 200 and videodecoder 300 may be configured to compare the ratio of the maximum andminimum of the horizontal and vertical gradients, denoted by R_(h,v) in(6) and the ratio of maximum and minimum of two diagonal gradients,denoted by R_(d1,d2) in (7) against each other with two thresholds t₁and t₂.

R _(h,v) =g _(h,v) ^(max) /g _(h,v) ^(min)  (6)

wherein g_(h,v) ^(max)=max(g_(h),g_(v)),g_(h,v) ^(min)=min(g_(h),g_(v)),

R _(d0,d1) =g _(d0,d1) ^(max) /g _(d0,d1) ^(min)  (7)

wherein g_(d0,d1) ^(max)=max(g_(d0),g_(d1)), g_(d0,d1)^(min)=min(g_(d0),g_(d1))

By comparing the detected ratios of horizontal/vertical and diagonalgradients, five direction modes, i.e., Dir_(b) within the range of [0,4] inclusive, are defined in equation (8). The values of Dir_(b) andtheir physical meaning are described in Table 1.

$\begin{matrix}{D = \left\{ {\begin{matrix}0 & {{{{{R_{h,v} \leq t_{1}}\&}\&}\mspace{11mu} R_{{d\; 0},{d\; 1}}} \leq t_{1}} \\1 & {{{{{{{{{R_{h,v} > t_{1}}\&}\&}R_{h,v}} > R_{{d\; 0},{d\; 1}}}\&}\&}\mspace{14mu} R_{h,v}} > t_{2}} \\2 & {{{{{{{{{R_{h,v} > t_{1}}\ \&}\&}\ R_{h,v}} > R_{{d\; 0},{d\; 1}}}\ \&}\&}\ R_{h,v}} \leq t_{2}} \\3 & {{{{{{{{{R_{{d\; 0},{d\; 1}} > t_{1}}\ \&}\&}\ R_{h,v}} \leq R_{{d\; 0},{d\; 1}}}\ \&}\&}\ R_{{d\; 0},{d\; 1}}} > t_{2}} \\4 & {{{{{{{{{R_{{d\; 0},{d\; 1}} > t_{1}}\ \&}\&}\ R_{h,v}} \leq R_{{d\; 0},{d\; 1}}}\ \&}\&}\ R_{{d\; 0},{d\; 1}}} \leq t_{2}}\end{matrix}.} \right.} & (8)\end{matrix}$

TABLE 1 Values of Direction and Their Physical Meaning Direction valuesphysical meaning 0 Texture 1 Strong horizontal/vertical 2horizontal/vertical 3 strong diagonal 4 diagonal

Video encoder 200 and video decoder 300 may be configured to calculatethe activity value Act as:

$\begin{matrix}{{Act}{{= {\sum\limits_{k = {i - 3}}^{i + 4}{\sum\limits_{l = {j - 3}}^{j + 4}\left( {V_{k,l} + H_{k,l}} \right)}}}.}} & (9)\end{matrix}$

Video encoder 200 and video decoder 300 may be configured to furtherquantize the value of Act to the range of 0 to 4, inclusive, and thequantized value is denoted as Â.

Quantization Process from Activity Value Act to Activity Index Â

The quantization process is defined as follows:

avg_var=Clip_post(NUM_ENTRY−1,(Act*ScaleFactor)>>shift);{circumflex over(A)}=ActivityToIndex[avg_var]

wherein NUM_ENTRY is set to 16, ScaleFactor is set to 64, shift is(4+internal coded-bitdepth), ActivityToIndex[NUM_ENTRY]={0, 1, 2, 2, 2,2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4}, function Clip_post(a, b) returns thesmaller value between a and b.

In total, video encoder 200 and video decoder 300 may be configured tocategorize each 4×4 luma block into one out of 25 (5×5) classes andassign an index to each 4×4 block according the value of Dir_(b) andAct_(b) of the block. Video encoder 200 and video decoder 300 may beconfigured to denote the group index by C and is set equal to 5Dir_(b)+Âwherein A is the quantized value of Act_(b).

Geometry Transformations

In some examples, for each category, video encoder 200 may be configuredto code and signal one set of filter coefficients and clipping values.To better distinguish different directions of blocks marked with thesame category index, four geometry transformations, including notransformation, diagonal, vertical flip, and rotation, may be used.

FIG. 6 is a conceptual diagram illustrating an example 5×5diamond-shaped filter support 600. An example of a 5×5 filter supportwith the three geometric transformations is depicted in FIG. 7,including a diagonal transformation 700, a vertical flip transformation710, and a rotation transform 720. Comparing FIG. 6 and FIG. 7, theformula forms of the three additional geometry transformations are asfollows:

Diagonal: f _(p)(k,l)=f(l,k),c _(D)(k,l)=c(l,k),

Vertical flip: f _(v)(k,l)=f(k,K−l−1),c _(V)(k,l)=c(k,K−l−1)

Rotation: f _(R)(k,l)=f(K−l−1,k),c _(R)(k,l)=c(K−l−1,k),  (10)

where K is the size of the filter, and 0≤k, 1≤K−1 are coefficientscoordinates, such that location (0,0) is at the upper left corner of ablock and location (K−1, K−1) is at the lower right corner of the block.When the diamond filter support is used, such as in the existingadaptive loop filter, the coefficients with a coordinate outside of thefilter support will be set to 0. One technique for indicating anddetermining the geometry transformation index is to derive the geometrytransformation index implicitly (e.g., without signaling) to avoidadditional overhead. In a Geometric Adaptive Loop Filter (GALF), videoencoder 200 and video decoder 300 may be configured to apply thetransformations to the filter coefficients f (k,l) depending on gradientvalues calculated for that block. The relationship between thetransformation and the four gradients calculated using equations (2)-(5)is described in Table 1. To summarize, the transformations are based onwhich one of two gradients (horizontal and vertical, or 45 degree and135 degree gradients) is larger. Based on the comparison, more accuratedirection information can be extracted. Therefore, different filteringresults could be obtained due to transformation, while the overhead offilter coefficients is not increased.

TABLE 2 Mapping of Gradient and Transformations. Gradient valuesTransformation g_(d2) < g_(d1) and g_(h) < g_(v) No transformationg_(d2) < g_(d1) and g_(v) < g_(h) Diagonal g_(d1) < g_(d2) and g_(h) <g_(v) Vertical flip g_(d1) < g_(d2) and g_(v) < g_(h) Rotation

Filter Information Signaling

One luma filter set includes filter information (including filtercoefficients and clipping values) for all 25 classes. In one example,video encoder 200 and video decoder 300 may use one or more fixedfilters to predict the filters for each class. In some examples, videoencoder 200 may be configured to signal a flag for each class toindicate whether this class uses a fixed filter as its filter predictor.If yes (e.g., fixed filter is used), the fixed filter information issignaled.

To reduce the number of bits required to represent the filtercoefficients, different classes can be merged. The informationindicating which classes are merged may be provided by sending, for eachof the 25 classes, an index i_(C). Classes having the same index i_(C)share the same filter coefficients that are coded. Video encoder 200 maybe configured to signal the mapping between classes and filters for eachluma filter set. Video encoder 200 and video decoder 300 may beconfigured to code the index i_(C) with a truncated binary binarizationmethod.

In some examples, video encoder 200 and video decoder 300 may beconfigured to predict a signaled filter from a previously signaledfilter.

Adaptive Parameter Set

In VTM-5.0, adaptive parameter sets (APSs) are used to carry adaptiveloop filter coefficients in bitstream. An adaptive parameter set mayalso be referred to as an adaptation parameter set. An APS is a syntaxstructure containing syntax elements that apply to slices as determinedby syntax elements found in slice headers. In some examples, an APS mayinclude a set of luma filters (e.g., adaptive loop filters for lumacomponents) or a chroma filter (e.g., adaptive loop filters for chromacomponents) or both. In some examples, video encoder 200 and videodecoder 300 may be configured to code indices of APSs used for thecurrent tile group in the corresponding tile group header.

Coding Tree Block (CTB)-Based Filter Set Switch

In VTM-5.0, video encoder 200 and video decoder 300 may be configured touse filters generated from previously-coded tile groups for a currenttile group in order to save overhead for filter signaling. Video encoder200 and video decoder 300 may be configured to determine, for a lumacoding tree block (CTB), a filter set from among fixed filter sets andfilter sets from APSs. Video encoder 200 may be configured to signal thefilter set index. Video decoder 300 may be configured to use a filterset for chroma CTBs from the same APS as the luma CTB. In a tile groupheader, slice header, and/or picture header, video encoder 200 maysignal the APSs used for luma and chroma CTBs of a current tile group.

The above-described features illustrate that some example adaptive loopfilter techniques lack flexibility to signal and determine filters forboth luma and chroma CTBs. In view of these shortcomings and lack offlexibility of the techniques discussed above, this disclosure describesthe following techniques that can improve upon the techniques above.Each of the techniques described below may be used independently or maybe used together in any combination. The techniques of the disclosuremay improve coding efficiency and/or reduce distortion indecoded/reconstructed video data by providing additional flexibility inselecting possible adaptive loop filters to apply for both luma andchroma components of video data. The techniques of the disclosuredescribed below refer to examples using coding tree blocks (CTBs).However, it should be understood that the techniques of this disclosureare applicable for use with any size blocks, include CTBs, coding units,prediction units, sub-blocks, groups of samples, and/or individualsamples.

In one example of the disclosure, video encoder 200 and video decoder300 may be configured to choose and/or determine filters (e.g., adaptiveloop filters) from among multiple APSs for chroma CTBs in one tilegroup, slice, and/or picture, chroma CTBs in groups of tiles or blocks,or chroma CTBs in slices. Video encoder 200 may be configured to signalone or more APS indices that are used for determining adaptive loopfilters for chroma blocks (e.g., CTBs, sub-blocks, groups of samples, ora sample) in a header, for example a slice header or tile group header.In one example, video encoder 200 may be configured to signal the numberof APSs used for chroma CTBs in the current slice or tile group and/orpicture. Then, video encoder 200 may be configured to signal the indicesof all used APSs. That is, video encoder 200 may be configured to signalindices for APSs for which video decoder 300 is to obtain an apply anadaptive loop filter.

For each chroma CTB where adaptive loop filtering is enabled, videoencoder 200 may be configured to signal the filter index or APS index.Video decoder 300 may be configured to decode the filter index and/orAPS index and apply the indicated adaptive loop filter. For example,video decoder 300 may obtain the adaptive loop filter for the chroma CTBthat is associated with the signaled APS index.

In one example, video encoder 200 and video decoder 300 may beconfigured to share the same set of APS indices or the same sets offilters for Cb and Cr chroma components. In another example, videoencoder 200 may be configured to signal a set of APS indices of filtersets used only for Cb component and may signal another set of APSindices that are only used for Cr component. Video decoder 300 maydecode the APS indices of filter sets used only for Cb component and maydecode the other set of APS indices that are only used for Cr componentand may the apply adaptive loop filters to the Cr and Cb componentsbased on the filters sets in the APSs indicated by the APS indices.

In some examples, video encoder 200 and video decoder 300 may beconfigured to apply an adaptive loop filter to Cb and Cr chromacomponents using separate filters. Instead of containing at most onechroma filter in an APS, an APS may have one filter (e.g., adaptive loopfilter) for Cb and another filter for Cr.

In one example, when both filters are signaled, video decoder 300 may beconfigured to predict one filter from another signaled filter. Videoencoder 200 may be configured to signal a flag to indicate whether theprediction is applied. If yes (e.g., prediction is applied), videoencoder 200 may be configured to signal the difference between thefilter and its predictor. Video decoder 300 may then add the signaleddifference to the predictor to obtain the filter.

In another example, video encoder 200 may be configured to signal a flagto indicate whether both filters for the Cr and Cb components are thesame.

In another example, video encoder 200 and video decoder 300 maydetermine a filter for a chroma CTB among multiple APSs. In one example,video encoder 200 and video decoder 300 may only use a filter from thesame color component for a chroma CTB. In another example, video encoder200 and video decoder 300 may use a filter for a chroma CTB from anycolor component.

In another example of the disclosure, video encoder 200 and videodecoder 300 may be configured to code an APS that includes multiple lumafilter sets. For example, video encoder 200 may be configured to encodea block of video data, and reconstruct the block of video data. Videoencoder 200 may then apply an adaptive loop filter to the reconstructedblock of the video data to create a filtered block of the video data. Inaccordance with the techniques of this disclosure, video encoder 200 maysignal an adaptation parameter sets (APS) in an encoded video bitstreamfor the block of the video data, wherein the APS includes a plurality ofadaptive loop filter sets, including the applied adaptive loop filter,for luma components of the block of the video data.

In a reciprocal manner, video decoder 300 may decode a block of thevideo data. Video decoder 300 may also receive an adaptation parametersets (APS) in an encoded video bitstream for the block of the videodata, wherein the APS includes a plurality of adaptive loop filter setsfor luma components of the block of the video data. Video decoder 300may then determine an adaptive loop filter from the plurality ofadaptive loop filter sets in the APS to apply to the decoded block ofthe video data, and apply the determined adaptive loop filter to thedecoded block of the video data to create a filtered block of the videodata. By using an APS that includes a plurality of adaptive loop filtersets, video encoder 200 may have more flexibility in indicating theactual filter set used, and the actual adaptive loop filter from thefilter set used, for different blocks of video data.

In one example, video encoder 200 and video decoder 300 may predict aluma filter set from one or multiple previously-coded filter sets. Forexample, video encoder 200 and video decoder 300 may predict an adaptiveloop filter set from the APS for the current block from one or morepreviously-decoded adaptive loop filter sets. Video encoder 200 may beconfigured to signal a flag to indicate whether the prediction isapplied. If yes (e.g., prediction is applied), video encoder 200 may beconfigured to signal the difference between the filter and itspredictor. Video decoder 300 may then add the signaled difference to thepredictor to obtain the filter.

When applying an adaptive loop filter, video encoder 200 may signal, fora CTB, which filter set and which merging table are used. A mergingtable may indicate the mapping between a class of filter (various blocksmay have different classifications) and the filter to be used or filterset to be applied. In this example, video encoder 200 may be configuredto signal one or more syntax elements that indicate the adaptive loopfilter set and a merging table, wherein the merging table indicates amapping between a class and the adaptive loop filter to be applied. In areciprocal fashion, to determine an adaptive loop filter from theplurality of the adaptive loop filter sets in an APS, video decoder 300may be configured to receive one or more syntax elements that indicatethe adaptive loop filter set and a merging table. Video decoder 300 maythen determine the adaptive loop filter from the indicated adaptive loopfilter set and merging table.

In one example, an APS could include one filter set and multiple mergingtables. That is, the plurality of adaptive loop filter sets in an APSare represented by a single adaptive loop filter set and multiplemerging tables. In another example, an APS could include one mergingtable and multiple filter sets. That is, the plurality of adaptive loopfilter sets in an APS are represented by a plurality of adaptive loopfilter sets and a single merging table. In another example, an APS couldinclude multiple filter sets and each set has one or more mergingtables. That is, the plurality of adaptive loop filter sets in an APSare represented by a plurality of adaptive loop filter sets, whereineach adaptive loop filter set of the plurality of adaptive loop filtersets is associated with one or more merging tables.

In another example of the disclosure, video encoder 200 and videodecoder 300 may be configured to code, for a luma CTB/block, multipleAPS indices to indicate which APSs the filters (e.g., adaptive loopfilters), merging table, and/or clipping values are from. For example,video encoder 200 may be configured to signal a plurality of APSs in theencoded video bitstream for a block of the video data, and signal aplurality of APS indices for the block of the video data, wherein eachrespective APS index of the plurality of APS indices indicates one ormore of which of the plurality of APSs from which to determine theadaptive loop filter, a merging table, or clipping values for the blockof the video data.

In a reciprocal fashion, video decoder 300 may be configured to receivea plurality of APSs in the encoded video bitstream for the block of thevideo data, receive a plurality of APS indices for the block of thevideo data, wherein each respective APS index of the plurality of APSindices indicates one or more of which of the plurality of APSs fromwhich to determine the adaptive loop filter, a merging table, orclipping values for the block of the video data, and determine theadaptive loop filter, the merge table, and clipping values based on theplurality of APS indices.

In one example, one APS index, of the plurality of APS indices,indicates which APS the filters are from, another APS index, of theplurality of APS indices, indicates which APS the clipping values arefrom, and another APS index, of the plurality of APS indices, indicatesthe APS index where the merging table is from.

In another example, one APS index, of the plurality of APS indices,indicates the APS where the filters (e.g., filter coefficients) andclipping values are from, and another APS index, of the plurality of APSindices, indicates the APS where the merging table is from.

In another example, one APS index, of the plurality of APS indices,indicates the APS where the filters (e.g., filter coefficients) andmerging table are from, and another APS index, of the plurality of APSindices, indicates the APS where the clipping values are from.

In another example, one APS index, of the plurality of APS indices,indicates the APS where the filters are from and another APS index, ofthe plurality of APS indices, indicates the APS where the clippingvalues and the merging table are from.

In another example, for a luma CTB/block, video encoder 200 and videodecoder 300 may re-use the coefficients and clipping values from oneAPS, and signal/decode a flag to indicate/determine whether the newmerging table is signaled. If the flag is true, a new merging tablesignaled for the CTB/block; otherwise, a merging table from an APS isused by video encoder 200 and video decoder 300.

In another example, for a luma CTB/block, video encoder 200 and videodecoder 300 may re-use the coefficients and merging table from one APS,and signal/decode a flag to indicate/determine whether the new clippingvalues are signaled. If the flag is true, new clipping values aresignaled for the CTB/block; otherwise, clipping values from an APS isused by video encoder 200 and video decoder 300.

In another example, for a luma CTB/block, video encoder 200 and videodecoder 300 may re-use the coefficients from one APS, and signal a flagto indicate whether the new clipping values are signaled. If the flag istrue, new clipping values are signaled for the CTB/block; otherwise,clipping values from an APS is used. In addition, a flag may be signaledto indicate whether the merging table is signaled. If the flag is true,a new merging table is signaled for the CTB/block; otherwise, themerging table from an APS is used by video encoder 200 and video decoder300.

In another example, for a luma CTB/block, video encoder 200 and videodecoder 300 may re-use merging tables from previously-coded CTBs/blocksand use filter coefficients from an APS.

In another example, for a luma CTB/block, video encoder 200 and videodecoder 300 may use filters from multiple filter sets. One flag could besignaled to indicate whether the filters are from the same filter set.If not, for each filter, video encoder 200 may signal which filter setthe filter is from.

In another example, for a CTB, video encoder 200 can use filtercoefficients and clipping values from different filter sets. One flagcould be signaled to indicate whether coefficients and clipping valuesare from the same filter sets. If not, the information about wherecoefficients and clipping values are obtained is signaled in a codedbitstream.

FIG. 8 is a flowchart illustrating an example method for encoding acurrent block. The current block may comprise a current CU. Althoughdescribed with respect to video encoder 200 (FIGS. 1 and 3), it shouldbe understood that other devices may be configured to perform a methodsimilar to that of FIG. 8.

In this example, video encoder 200 initially predicts the current block(350). For example, video encoder 200 may form a prediction block forthe current block. Video encoder 200 may then calculate a residual blockfor the current block (352). To calculate the residual block, videoencoder 200 may calculate a difference between the original, unencodedblock and the prediction block for the current block. Video encoder 200may then transform and quantize coefficients of the residual block(354). Video encoder 200 may then reconstruct the block and apply anadaptive loop filter (process A). Process A will be described in moredetail with reference to FIG. 9. Next, video encoder 200 may scan thequantized transform coefficients of the residual block (356). During thescan, or following the scan, video encoder 200 may entropy encode thecoefficients (358). For example, video encoder 200 may encode thecoefficients using CABAC. Video encoder 200 may then output the entropycoded data of the block (360).

FIG. 9 is a flowchart illustrating an example encoding method of thedisclosure in more detail. One or more structural components of videoencoder 200 may be configured to perform the techniques of FIG. 9,including filter unit 216.

In one example of the disclosure, video encoder 200 may be configured toencode a block of the video data (900). For example, video encoder 200may encode the block of video data using the method of FIG. 8. Thenvideo encoder 200 may perform process A from FIG. 8. For example, videoencoder 200 may reconstruct the block of the video data (902), andfilter unit 216 of video encoder 200 may apply an adaptive loop filterto the reconstructed block of the video data to create a filtered blockof the video data (904). Video encoder 200 may also signal an adaptationparameter sets (APS) in an encoded video bitstream for the block of thevideo data, wherein the APS includes a plurality of adaptive loop filtersets, including the applied adaptive loop filter, for luma components ofthe block of the video data (906).

In one example, video encoder 200 may be configured to signal one ormore syntax elements that indicate the adaptive loop filter set and amerging table, wherein the merging table indicates a mapping between aclass and the adaptive loop filter to be applied. In one example, theplurality of adaptive loop filter sets are represented by a singleadaptive loop filter set and multiple merging tables. In anotherexample, the plurality of adaptive loop filter sets are represented by aplurality of adaptive loop filter sets and a single merging table. Inanother example, the plurality of adaptive loop filter sets arerepresented by a plurality of adaptive loop filter sets, wherein eachadaptive loop filter set of the plurality of adaptive loop filter setsis associated with one or more merging tables.

In another example, video encoder 200 may be configured to signal aplurality of APSs in the encoded video bitstream for the block of thevideo data, and signal a plurality of APS indices for the block of thevideo data, wherein each respective APS index of the plurality of APSindices indicates one or more of which of the plurality of APSs fromwhich to determine the adaptive loop filter, a merging table, orclipping values for the block of the video data.

FIG. 10 is a flowchart illustrating an example method for decoding acurrent block of video data. The current block may comprise a currentCU. Although described with respect to video decoder 300 (FIGS. 1 and4), it should be understood that other devices may be configured toperform a method similar to that of FIG. 10.

Video decoder 300 may receive entropy coded data for the current block,such as entropy coded prediction information and entropy coded data forcoefficients of a residual block corresponding to the current block(370). Video decoder 300 may entropy decode the entropy coded data todetermine prediction information for the current block and to reproducecoefficients of the residual block (372). Video decoder 300 may predictthe current block (374), e.g., using an intra- or inter-prediction modeas indicated by the prediction information for the current block, tocalculate a prediction block for the current block. Video decoder 300may then inverse scan the reproduced coefficients (376), to create ablock of quantized transform coefficients. Video decoder 300 may theninverse quantize and inverse transform the coefficients to produce aresidual block (378). Video decoder 300 may ultimately decode thecurrent block by combining the prediction block and the residual block(380). Video decoder 300 may apply an adaptive loop filter to thedecoded block (process B). Process B will be described in more detailwith reference to FIG. 11.

FIG. 11 is a flowchart illustrating an example decoding method of thedisclosure in more detail. One or more structural components of videodecoder 300 may be configured to perform the techniques of FIG. 9,including filter unit 312.

In one example, video decoder 300 may be configured to decode a block ofthe video data (950). For example, video decoder 300 may decode theblock of video data using the method of FIG. 10. Then video decoder 300may perform process B from FIG. 10. For example, video decoder 300 mayreceive an adaptation parameter sets (APS) in an encoded video bitstreamfor the block of the video data, wherein the APS includes a plurality ofadaptive loop filter sets for luma components of the block of the videodata (952), and determine an adaptive loop filter from the plurality ofadaptive loop filter sets in the APS to apply to the decoded block ofthe video data (954). Video decoder 300 may then apply the determinedadaptive loop filter to the decoded block of the video data to create afiltered block of the video data (956).

In one example, video decoder 300 may be configured to predictive anadaptive loop filter set from the APS for the current block from one ormore previously-decoded adaptive loop filter sets.

In another example, to determine the adaptive loop filter from theplurality of the adaptive loop filter sets in the APS, video decoder 300may be configured to receive one or more syntax elements that indicatethe adaptive loop filter set and a merging table, wherein the mergingtable indicates a mapping between a class and the adaptive loop filterto be applied to the decoded block of the video data, and determine theadaptive loop filter from the indicated adaptive loop filter set andmerging table.

In one example, the plurality of adaptive loop filter sets arerepresented by a single adaptive loop filter set and multiple mergingtables. In another example, the plurality of adaptive loop filter setsare represented by a plurality of adaptive loop filter sets and a singlemerging table. In another example, the plurality of adaptive loop filtersets are represented by a plurality of adaptive loop filter sets,wherein each adaptive loop filter set of the plurality of adaptive loopfilter sets is associated with one or more merging tables.

In another example, video decoder 300 may be configured to receive aplurality of APSs in the encoded video bitstream for the block of thevideo data, receive a plurality of APS indices for the block of thevideo data, wherein each respective APS index of the plurality of APSindices indicates one or more of which of the plurality of APSs fromwhich to determine the adaptive loop filter, a merging table, orclipping values for the block of the video data, and determine theadaptive loop filter, the merge table, and clipping values based on theplurality of APS indices.

Other examples of the disclosure are described below.

Example 1

A method of coding video data, the method comprising: reconstructing acoding tree block; determining an adaptive loop filter for the codingtree block according to one or more techniques of this disclosure; andapplying the adaptive loop filter to the reconstructed coding treeblock.

Example 2

The method of Example 1, wherein the coding tree block is a chromacoding tree block, and wherein determining the adaptive loop filter forthe chroma coding tree block comprises: determining the adaptive loopfilter for the chroma coding tree block from among multiple adaptiveparameter sets.

Example 3

The method of Example 2, wherein the chroma coding tree block includes aCr coding tree block and a Cb coding tree block, and wherein determiningthe adaptive loop filter for the chroma coding tree block comprises:determining the same adaptive loop filter for both the Cr coding treeblock and the Cb coding tree block.

Example 4

The method of Example 2, wherein the chroma coding tree block includes aCr coding tree block and a Cb coding tree block, and wherein determiningthe adaptive loop filter for the chroma coding tree block comprises:determining different adaptive loop filters for both the Cr coding treeblock and the Cb coding tree block.

Example 5

The method of Example 1, further comprising: coding one or more ofmultiple adaptive parameter sets, wherein a first adaptive parametersets include multiple adaptive loop filter sets for luma components.

Example 6

The method of Example 5, further comprising: predicting an adaptive loopfilter set for a luma component from previously-coded filter sets.

Example 7

The method of Example 1, further comprising: coding an index to a firstadaptive parameter set from among multiple adaptive filter sets, whereinthe index indicates the first adaptive parameter set from which todetermine one or more of an adaptive loop filter, a merging table, orclipping values.

Example 8

The method of Example 1, wherein the coding tree block is a luma codingtree block, and wherein determining the adaptive loop filter for theluma coding tree block comprises: re-using filter coefficients for theadaptive loop filter from a previously-coded adaptive parameter set.

Example 9

The method of Example 8, further comprising: coding clipping values forthe luma coding tree block.

Example 10

The method of Example 1, wherein the coding tree block is a luma codingtree block, and wherein determining the adaptive loop filter for theluma coding tree block comprises: re-using a merging table for theadaptive loop filter from a previously-coded adaptive parameter set.

Example 11

The method of Example 1, further comprising: displaying a picture thatincludes the reconstructed coding tree block

Example 12

The method of any of Examples 1-11, wherein coding comprises decoding.

Example 13

The method of any of Examples 1-11, wherein coding comprises encoding.

Example 14

A device for coding video data, the device comprising one or more meansfor performing the method of any of Examples 1-13.

Example 15

The device of Example 14, wherein the one or more means comprise one ormore processors implemented in circuitry.

Example 16

The device of any of Examples 14 and 15, further comprising a memory tostore the video data.

Example 17

The device of any of Examples 14-16, further comprising a displayconfigured to display decoded video data.

Example 18

The device of any of Examples 14-17, wherein the device comprises one ormore of a camera, a computer, a mobile device, a broadcast receiverdevice, or a set-top box.

Example 19

The device of any of Examples 14-18, wherein the device comprises avideo decoder.

Example 20

The device of any of Examples 14-19, wherein the device comprises avideo encoder.

Example 21

A computer-readable storage medium having stored thereon instructionsthat, when executed, cause one or more processors to perform the methodof any of Examples 1-13.

Example 22

Any combination of techniques described in this disclosure.

It is to be recognized that depending on the example, certain acts orevents of any of the techniques described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of thetechniques). Moreover, in certain examples, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the terms “processor” and “processingcircuitry,” as used herein may refer to any of the foregoing structuresor any other structure suitable for implementation of the techniquesdescribed herein. In addition, in some aspects, the functionalitydescribed herein may be provided within dedicated hardware and/orsoftware modules configured for encoding and decoding, or incorporatedin a combined codec. Also, the techniques could be fully implemented inone or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. An apparatus configured to decode video data, theapparatus comprising: a memory configured to store the video data; andone or more processors in communication with the memory, the one or moreprocessors configured to: decode a block of the video data; receive anadaptation parameter sets (APS) in an encoded video bitstream for theblock of the video data, wherein the APS includes a plurality ofadaptive loop filter sets for luma components of the block of the videodata; determine an adaptive loop filter from the plurality of adaptiveloop filter sets in the APS to apply to the decoded block of the videodata; and apply the determined adaptive loop filter to the decoded blockof the video data to create a filtered block of the video data.
 2. Theapparatus of claim 1, wherein the one or more processors are furtherconfigured to: predict an adaptive loop filter set from the APS for theblock from one or more previously-decoded adaptive loop filter sets. 3.The apparatus of claim 1, wherein to determine the adaptive loop filterfrom the plurality of the adaptive loop filter sets in the APS, the oneor more processors are further configured to: receive one or more syntaxelements that indicate the adaptive loop filter set and a merging table,wherein the merging table indicates a mapping between a class and theadaptive loop filter to be applied to the decoded block of the videodata; and determine the adaptive loop filter from the indicated adaptiveloop filter set and merging table.
 4. The apparatus of claim 1, whereinthe plurality of adaptive loop filter sets are represented by a singleadaptive loop filter set and multiple merging tables.
 5. The apparatusof claim 1, wherein the plurality of adaptive loop filter sets arerepresented by a plurality of adaptive loop filter sets and a singlemerging table.
 6. The apparatus of claim 1, wherein the plurality ofadaptive loop filter sets are represented by a plurality of adaptiveloop filter sets, wherein each adaptive loop filter set of the pluralityof adaptive loop filter sets is associated with one or more mergingtables.
 7. The apparatus of claim 1, wherein the one or more processorsare further configured to: receive a plurality of APSs in the encodedvideo bitstream for the block of the video data; receive a plurality ofAPS indices for the block of the video data, wherein each respective APSindex of the plurality of APS indices indicates one or more of which ofthe plurality of APSs from which to determine the adaptive loop filter,a merging table, or clipping values for the block of the video data; anddetermine the adaptive loop filter, the merge table, and clipping valuesbased on the plurality of APS indices.
 8. The apparatus of claim 1,further comprising: a display configured to display a picture thatincludes the filtered block of the video data.
 9. A method for decodingvideo data, the method comprising: decoding a block of the video data;receiving an adaptation parameter sets (APS) in an encoded videobitstream for the block of the video data, wherein the APS includes aplurality of adaptive loop filter sets for luma components of the blockof the video data; determining an adaptive loop filter from theplurality of adaptive loop filter sets in the APS to apply to thedecoded block of the video data; and applying the determined adaptiveloop filter to the decoded block of the video data to create a filteredblock of the video data.
 10. The method of claim 9, further comprising:predicting an adaptive loop filter set from the APS for the block fromone or more previously-decoded adaptive loop filter sets.
 11. The methodof claim 9, wherein determining the adaptive loop filter from theplurality of the adaptive loop filter sets in the APS comprises:receiving one or more syntax elements that indicate the adaptive loopfilter set and a merging table, wherein the merging table indicates amapping between a class and the adaptive loop filter to be applied tothe decoded block of the video data; and determining the adaptive loopfilter from the indicated adaptive loop filter set and merging table.12. The method of claim 9, wherein the plurality of adaptive loop filtersets are represented by a single adaptive loop filter set and multiplemerging tables.
 13. The method of claim 9, wherein the plurality ofadaptive loop filter sets are represented by a plurality of adaptiveloop filter sets and a single merging table.
 14. The method of claim 9,wherein the plurality of adaptive loop filter sets are represented by aplurality of adaptive loop filter sets, wherein each adaptive loopfilter set of the plurality of adaptive loop filter sets is associatedwith one or more merging tables.
 15. The method of claim 9, furthercomprising: receiving a plurality of APSs in the encoded video bitstreamfor the block of the video data; receiving a plurality of APS indicesfor the block of the video data, wherein each respective APS index ofthe plurality of APS indices indicates one or more of which of theplurality of APSs from which to determine the adaptive loop filter, amerging table, or clipping values for the block of the video data; anddetermining the adaptive loop filter, the merge table, and clippingvalues based on the plurality of APS indices.
 16. The method of claim 9,further comprising: displaying a picture that includes the filteredblock of the video data.
 17. An apparatus configured to encode videodata, the apparatus comprising: a memory configured to store the videodata; and one or more processors in communication with the memory, theone or more processors configured to: encode a block of the video data;reconstruct the block of the video data; apply an adaptive loop filterto the reconstructed block of the video data to create a filtered blockof the video data; and signal an adaptation parameter sets (APS) in anencoded video bitstream for the block of the video data, wherein the APSincludes a plurality of adaptive loop filter sets, including the appliedadaptive loop filter, for luma components of the block of the videodata.
 18. The apparatus of claim 17, wherein the one or more processorsare further configured to: signal one or more syntax elements thatindicate the adaptive loop filter set and a merging table, wherein themerging table indicates a mapping between a class and the adaptive loopfilter to be applied.
 19. The apparatus of claim 17, wherein theplurality of adaptive loop filter sets are represented by a singleadaptive loop filter set and multiple merging tables.
 20. The apparatusof claim 17, wherein the plurality of adaptive loop filter sets arerepresented by a plurality of adaptive loop filter sets and a singlemerging table.
 21. The apparatus of claim 17, wherein the plurality ofadaptive loop filter sets are represented by a plurality of adaptiveloop filter sets, wherein each adaptive loop filter set of the pluralityof adaptive loop filter sets is associated with one or more mergingtables.
 22. The apparatus of claim 17, wherein the one or moreprocessors are further configured to: signal a plurality of APSs in theencoded video bitstream for the block of the video data; and signal aplurality of APS indices for the block of the video data, wherein eachrespective APS index of the plurality of APS indices indicates one ormore of which of the plurality of APSs from which to determine theadaptive loop filter, a merging table, or clipping values for the blockof the video data.
 23. The apparatus of claim 17, further comprising: acamera configured to capture a picture that includes the block of thevideo data.
 24. A method of encoding video data, the method comprising:encoding a block of the video data; reconstructing the block of thevideo data; applying an adaptive loop filter to the reconstructed blockof the video data to create a filtered block of the video data; andsignaling an adaptation parameter sets (APS) in an encoded videobitstream for the block of the video data, wherein the APS includes aplurality of adaptive loop filter sets, including the applied adaptiveloop filter, for luma components of the block of the video data.
 25. Themethod of claim 24, further comprising: signaling one or more syntaxelements that indicate the adaptive loop filter set and a merging table,wherein the merging table indicates a mapping between a class and theadaptive loop filter to be applied.
 26. The method of claim 24, whereinthe plurality of adaptive loop filter sets are represented by a singleadaptive loop filter set and multiple merging tables.
 27. The method ofclaim 24, wherein the plurality of adaptive loop filter sets arerepresented by a plurality of adaptive loop filter sets and a singlemerging table.
 28. The method of claim 24, wherein the plurality ofadaptive loop filter sets are represented by a plurality of adaptiveloop filter sets, wherein each adaptive loop filter set of the pluralityof adaptive loop filter sets is associated with one or more mergingtables.
 29. The method of claim 24, further comprising: signaling aplurality of APSs in the encoded video bitstream for the block of thevideo data; and signaling a plurality of APS indices for the block ofthe video data, wherein each respective APS index of the plurality ofAPS indices indicates one or more of which of the plurality of APSs fromwhich to determine the adaptive loop filter, a merging table, orclipping values for the block of the video data.
 30. The method of claim24, further comprising: capturing a picture that includes the block ofthe video data.